Method of making hollow shapes



Sept. 9, 1952 M. s.- ROUSH ETAL 2,609,576

METHOD F'MAKING HOLLOW SHAPES Filed Dec 6. 1949 E Lj [Tu En 22:7 r5

4 6. 8008/1 CLYDE C CLARK f Ff/g5.

Patented Sept. 9, 1952 METHOD OF MAKING HOLLOW sHArEs Milton S. Roush,Painesville, and Clyde 0. Clark,

Willoiighby Ohio, assignors to Thompson Products, Ina, Cleveland, Ohio,a corporation of Ohio i l eradication December 6, 1.9149, S r al, No;13. .1 19

The present inventionirelates to a method for making hollow shapes,particularly hollow castings of complex configuration.

In the fabrication of variouselements for internal combustion or "gasengines. it is often desirable to provide such elements with hollowinteriors into whichsuitable coolant materials may be inserted for thepurpose of enabling the 9 Claims. (531. 22-200) element towithstand"'theeffects of high temperature operating conditions. One suchelement is'a poppet valve in the exhaust system of an internalcombustion engine, wherein it is customaryto provide the poppetvalvehead and stem with a hollow interior intowhich metallic sodium or othercoolant material is. incorporated. Other elements in which use isrnadeof hollow 'body structure are the blhdfi q and turbine buckets used injet propulsion engines. v An object of the present invention is toprovide a method for making such hollow shapes, and in particular formaking hollow castings having a uniform wall thickness. e 7 Anotherobjec of the prese t nven io is t provide a method for. casting acorrosion resist ant casing about a preformed core while providing meansfor quickly and eillciently removing the core structure after thecasting has been completed. A 7

Still another object of the present invention is to provide a method formaking castings having hollow interiors, with the dimensions of the voidspace within theicastingbeing-controllable to precise limits. 1 l fOther objects and features. of the present invention will be apparent tothose skilled in the art from the following description taken inconnection with the attached sheetoi drawings.

As one feature of the present invention, we have provided a method forproducing hollow shapes by forming a core by powder metallurgy techniquehaving the configuration oi the void space desired in the ultimateshape, forming the body or casingiolround such a preformed corestructure, and dissolvingthe powder metal core out of the interior ofthe casing by means of a suitable reagent. The core material and .thebody metal are so selected that-the core metal will not be melted by thebody. metal which is cast upon it, although it may have a lower meltingpoint than the body metal. The core is preserved, irrespective of itsmelting point, if the molten body metal is not permitted to heat thecore close to its melting point, so that no buckling of the core occursduring the casting operation. In addition, it is preferred that the bodymetal be such that it does not wet the'oore, thus prevent ng orminimizing the amount of infiltration or body metal I in thecorestructor n dditiong the c re 1.. such math. i capabl of dissolutionby k id 1 a .11 time, while the, body metal is ofv such ,eom'posie tionthat it is. extremel resistant to attack by such reagents, thusfacilitating removal of the core from the finished castin While thepowder metal core preferably has a high degree of porosity itinay beginfiltered with an easily dissolved metal, such "as coppei to providestrength andihcre'ase its heat conductivity. The iniil'tered core, 01.purposes of thisinvention, may be .e'onsid 'd porous in that theinfiltrant filling" the void. nstead of'air is easily attack d ythdissolving I it-tl a to o n up the channels or voids tho-powder metalcore.

' By using a powder metal .c'ore in'a casting procedure Severaladvantages [are to, be realized. First of all, the poro ity. ofthe'eonipact is Such that it is easily 'penetrable by'flthe acid orother reagent and isquickly dissolved frdm the interior oi the castarticle; rhenitoo the. Core is C article, with tho @0129 iii pnceytoaifinal shape before the rmovalfo the poro s core. This latter ififi lreisseiitial in t fabrication of complex shapes such as the airgfoilsections ofturbihe buckets. h 'H The core metal preferably Iiised' inconi ition with this invention is o'wdeifed ire Lcoinpact which hasbeeri'co'r il hcfed .to the vshapedesired in the ultimate hollow sectionor the casting, and sintered at the a propriatetrsnsrormationjtemperaturcs according to .conventional powder metallur ytechniques.- densi y Oi Sl c compact will normauy' 1b "on th order of.50 to of the density ofsql d irohp The iron par, ticles initiallyhave-Iasiz'e'of irom ahout 8;) to 325 mesh, andar'e compacted atpressures from about 6 to SOItons 'er square .i ichl The temperature ofsintel gv preferablyat theaflp agamma transformatio int? or iron at at1600' F.) although temperaturesiup to 20010 may be used. t

.A s ere imet l core'iof 911 t pe diss nted-has the further advantage'tha fclfifiyfil'y .flmfi pressib'le, so that nocrackms or ,th' oastioccurs upon cooling; V I I y In addition to}, ou'fslfctallcoinpact,suitable, core materials may be 'rhade'from compactedcopper, powder, aluminum powder, or titanium powder. Where theoutercasing' of; the casting is to consist of a relatively low meltingplastic use of powder metallurgy technique to form an easily dissolvedcore, and the use of a casting technique which will not melt vor bucklethe powder metal core.

The casting of hollow articles maybe facili: tated by this invention,since the porous core permits evacuation of the mold cavity through thecore by merely connecting the core to a'source of vacuum. Further, theinvention makes pos-- sible the use of powder metallurgy technique toform molds which can be dissolved off of the casting.

Where the finished article is to be used under conditions of hightemperature corrosion, the preferred body metal which'forms the exteriorcasing is preferably a corrosionresistant alloy of the type knowncommercially as'the Vitallium alloys. A typical composition for such analloy includes 30% chromium, 6 molybdenum, and the balance cobalt.Portions of the cobalt may be replaced by nickel, and portions of themolybdenum content re laced by tungsten. Other suitable alloys are those"of the Stellite series, which contain about 45-55% cobalt, 28-32%chromium, 10-15% tungsten, and about 2.5% carbon. These alloys havemelting points around 2450 F. These alloys are extremely resistant tolead oxide corrosion and are substantially unattacked by mineral acidssuchv as hydrochloric acid and nitric acid. In addition, they do, notwet the porous infiltration of the latter is avoided during the castingoperation.

Another method forforming the body of the article about the preshapedporous core consists ure 3 and including the other half of the mold;

.40 iron compacts to any substantial degree. so that in chromizing thepowder metal compact in a conventional chromizingprocedure.

This procedure consists in packing the material to be chromized in aclosed container with chromium powder and then heating to a temperaturebetween about 1800 and 2600 F. in an atmosphere of hydrogen.

The chromizing operation is carried out until such time that aself-sustaining outer shell or casing is provided about the porous metalcore. The thickness of the casing may be conveniently controlled bycontrolling the time or temperature of the chromizing operation. Thecasing may be made more dense by coining the sintered porous compact toa high density, or by sintering the same in a carburizing atmosphere.

The case which is produced in such a chromizing operation is essentiallya iron-chromium alloy having a diminishing chromium concentrationthroughout the body of the case. The outside layer of the case has beenfound to contain approximately 60% chromium, while the layer immediatelyadjacent to the porous compact has a chromium concentration ofapproximately 10%.

A more detailed description of the present invention will be made inconjunction with the description of the attached sheet of drawings, inwhich:

Figure 1 is a vertical cross-sectional view of a poppet valve castingaround a porous compact and Figure 5 is a vertical sectional view of ahollow turbine blade'composed of a metal built up on the core as acoating, by chromizing or the like.

As shown on the drawings:

In Figure 1, the reference numeral 10 denotes generally a, permanentmold consisting of two symmetrical halves such as H mating at apartingline, While a permanent mold is used for purposes ofillustration, it'will be understood that sand and plaster molds may beused where appropriate. Disposed'within therecessof the mold is a corecomprising a powderjmetal compact I2 shaped to the configuration desiredin the void space of the finished article, as described previ ously. Inthe illustration shown in Figure 1, the compact core l2 forms the neckand stem portions of a poppet valve assembly. The base of the compactedcore 12 is provided with an appendage which forms a core print '-l3;heldbetween appropriate recesses formed in the mating spaces of the mold i0,therebyholding the compacted core 12' within the mold lll. I

Asshown in-Figure l, a castingalloy which is to form thebodybf thepoppet valve is cast around the compacted'core l2 while the same is heldwithin the mold Hi. The body thus formed consists of an arcuate valvehead portion 15, an elongated stem portion i6, and a base portion ll. Itwill be noted that the base portion ll terminates short of the lowerbase of the mold In so that the core printl3 extends downwardly beyondthe limit of the base ll. A sprue portion 3 is also formed during thecasting operation.

When the assembly shown in Figure 1 is removed from the mold 10, thesprue portion 18 may be trimmed off, and the core I! removed from thecast body. This removal may be most conveniently effected by immersingthe cast valve into a solution of acidor other reagent capable ofremoving the porous core without substantially afiecting the body metal.'By virtue of the high degree of Y porosity of the core, the acid willbe quickly soaked up into the interstices of the core, and dissolutionwill be efiected very rapidly. The dissolved core may then be drainedfrom the interior ofthe valve structure through the opening provided bythe dissolution of the core print l3. Coolant material, such as sodiumI9 is then inserted through the opening provided by the core printportion I3 to partially fill the hollow interior of thevalve. To" closethe sodium filled chamber, a tapered plug I9 is inserted into the valvebase ll, and a cap 20 applied to the valve base l'las by means ofwelding.

Figures 3 and 4 illustrate a method similar to that described inconnection with Figures 1 and 2 for the production of a hollow castturbine bucket. As shown in Figure 3, a'porous core 2! is providedwithin the mating faces of an appropriately recessed mold 22, the core2| having a core print 23 extending'therefrom and serving to secure andto index the position of the core 2! isjeastsmuna it, "theco'ri'ffi is-'provide'djvwi'ith a coating 25.' 'Ihe'coating' may be applied by'casting, dipping, spraying, electroplating and similar techniques wellknown by those skilled in the art., This coating. may consist of ametallic coating, such as chromium or nickel, or may suitably becomposed of a vitrified ceramic compositione The casting metal is pouredinto the mold structure, around the preformed porous core 2|, I

to define an air foil section 21 and a root portion 28 of the finishedbucket. A sprue 29 remains, partially enclosing the core print 23 of thecore 2!.

The cast assembly may then be removed from the mold and coined orotherwise shaped to its final configuration. To insure a proper degreeof rigidity in the air-foil section during such finishing operation, itis preferable to leave the compacted core 2| within the cast body untilthe final shape is attained. The compacted core may then be removed byacid dissolution in-a manner already described, leaving the body metaland as much of the coating 26 as has not been dissolved by the acid.

It will be noted that in the process described above the casting of thearticle may be accomplished while leaving only a small outlet aperturefor the removal of the porous core. Thus, an element may be cast toclose tolerances initially and the closure of such outlet is all that isnecessary to provide a sealed hollow chamber extending throughout thecast body.

As shown in Figure 5, a hollow turbine blade 39 can be produced by thisinvention by forming the blade metal as a coating on a core such as thecore 24 of Fig. 3 and by dissolving out the core. No casting operationis then involved. The blade 38, if formed by chromizing, will have theexact shape and dimensions of the core, since the chromized layer isactually created by replacement of the core surface and an externalbuilding up of an added layer is not. involved. It will be of controlledporosity, depending upon the porosity of the core, but it will be lessporous than the core on which it is formed. The porous blade in use in aturbo-jet engine can be sweat-cooled by oozing alcohol or some othervaporizable material through the blade.

From the foregoing it will be appreciated that we have herein provided aconvenient method for producing articles having complex interior voids.The articles so produced have uniform wall thicknesses and may beprecision cast to close tolerances.

It will be evident that various modifications and variations may beefiected without departing from the scope of the novel concepts of thepresent invention.

We claim as our invention:

1. A method of producing hollow articles which comprises compactingpowdered metal into a core having the configuration desired in thehollow portion of the finished article, casting metal to form the bodyof the article about said core without melting or buckling the core, anddissolving out said core with a reagent capable of dissolving ew-tea-materiaL .fi u fish, fa ou a {said core, anddissolvih out said ironcore ima ,3. The-method of making hollow castings' whichcomprises-forming a core'comprising a porous sintered powdered aluminumcompact into the configuration desired in-thetfhollow portion of thefinished article, castingibody metal-about said core; and dissolving outsaid aluminum core with an alkali.

4. The method of making hollow castings which comprises forming a corecomprising a porous metal compact into a shape having the configurationof the hollow portion of the finished casting and also having anappendage forming a core print, supporting said core in a mold by meansof said core print, casting an acid resistant body metal about said corewhile supported in said mold, removing the cast article from the mold,and dissolving the core from said cast article through said core print.

5. The method of making hollow castings which comprises forming a corecomprising a sintered powdered ferrous metal compact into a shape.

having the-configuration of the hollow portion of the finished casting,coating the core with a metal coating, casting an acid-resistant bodymetal about said coating, and dissolving out said core with an acidcapable of dissolving said ferrous core without substantially afiectingthe body metal.

6. The method of making hollow castings which comprises forming a corecomprising a sintered powdered ferrous metal compact into a shape havingthe configuration of the hollow portion of the finished casting, coatingthe core with a refractory coating, casting an acid resistant body metalabout said coated core, and dlSSOlViIlg out said core with an acidcapable of dissolving said ferrous core without substantially affectingthe body metal.

7. The method of making hollow castings which comprises forming a corecomprising a sintered powdered ferrous metal compact into a shape havingthe configuration of the hollow portion of the finished casting,chromizing the core, casting an acid resistant body metal about thechromized core, and dissolving out said core in an acid capable ofdissolving the core without substantially affecting the body metal.

8. The method of making a hollow metal article which comprises forming acore comprising a porous sintered powdered metal compact, casting thebody metal of the article about said core, and dissolving out said corewith a reagent'capable of dissolving the core Without substantiallyaffecting the body metal.

9. The method of making a hollow metal article which comprises forming acore comprising a porous sintered powdered metal compact, casting thebody metal of the article about said core, deforming the resultingarticle with the core in place to produce an irregularly shaped member,and thereafter dissolving out said core with a reagent capable ofdissolving the, core without substantially affecting the body metal.

MILTON S. ROUSH.

CLYDE C. CLARK.

(References on following page) REFERENCES CITED The following referencesare of record in the file of this patent:

UNITED STATES PATENTS Number r I V; FQREIGN :PATENTS Sept. 22, 1932 Apr.23,1937 Nov. 1, 1945 Gray et'al. Qct. 20, 1942 V 5 l 16999 7 5

